A conventional system for flow measurement employs a turbine assembly including a multi-blade rotary turbine. The turbine assembly is mounted in a fluid flow path such that the fluid flow through the turbine assembly and across the turbine blades causes the turbine to rotate. The rate of rotation of the turbine is linearly related to the rate of flow through the turbine assembly. Thus, if the rate of rotation of the turbine is known, the flow rate can be calculated accurately.
An infrared rotation sensor is typically used to detect the rate of rotation of the flow sensor turbine. A transparent window is provided in a sidewall of the turbine assembly. The infrared rotation sensor is attached to the turbine assembly and detects the passing of the turbine blades past the window in the sidewall thereof. At each pass of a rotary turbine blade, the infrared flow sensor may provide a signal pulse to a microprocessor controlled flow monitor, which calculates the flow rate through the turbine assembly from the calculated rate of rotation of the rotary turbine. The calculated flow rate may then be displayed, in either English or metric units, to a user of the flow rate measurement system. The microprocessor based flow monitor may also be programmed to provide various additional information and features based on the calculated flow rate. Such additional information and features might include, for example, calculation of cumulative flow through the turbine assembly, and the display of high or low flow rate alarms.
Flow sensor turbines employed in this conventional method of flow measurement exhibit inherent linear behavior over a range of flow rates. Thus, flow measurement systems employing such turbines typically do not require linearization for accurate flow measurement readings to be obtained. However, such flow measurement systems must be calibrated for the flow sensor turbine assembly which is employed. For example, the flow measurement system must typically be calibrated to the number of signal pulses, corresponding to each pass of the rotary turbine blade past the rotation sensor, per gallon of fluid flowing through the turbine assembly.
In typical prior art flow sensor turbine assemblies, the turbine blades are mounted on a turbine shaft bearing, which is typically made of bronze. The turbine shaft bearing is rotatably mounted on a turbine shaft. The turbine shaft is mounted in the turbine assembly between end thrust bearings made, e.g., of tungsten carbide. In conventional turbine assemblies, the end thrust bearings may be implemented as tungsten carbide ball bearings. As fluid flows through the turbine assembly, the turbine blades are rotated by the fluid flow, and the turbine shaft bearing rotates against the end thrust bearings. In order to maintain the linear behavior of the turbine assembly, it is important that the friction between the turbine shaft bearing and the end thrust bearings in the turbine assembly remain low and constant across a wide range of flow rates through the turbine assembly. Moreover, in order to maintain accurate flow measurement, without repeated recalibration, it is important that the friction between the turbine shaft bearing and the thrust bearings remain low and constant over extended periods of use of the turbine assembly. It has been found, however, that conventional tungsten carbide thrust bearings do not provide a low and consistent friction level over a wide range of flow rates. Also, such conventional thrust bearings can exhibit wear, and thus friction variations, after extended use of the turbine assembly, thereby affecting flow measurement accuracy, and requiring repeated recalibration of the flow sensor system.
As just discussed, a conventional flow sensor system typically employs an infrared rotation sensor placed adjacent to a window in the side wall of a turbine assembly to detect the rotation of the turbine blades which are mounted within the assembly. An infrared sensor is used because infrared frequency radiation is better able to penetrate a wide variety of fluid materials for which flow measurements may be made by flowing the fluid through the turbine assembly. However, it may be desirable to measure the flow of fluids which are completely or partially opaque to or otherwise mask infrared frequencies, such as high temperature materials. In such a case, a conventional infrared rotation sensor may not be used to make an accurate flow measurement.